The present disclosure relates generally to a system and method for calculating the state of charge (SOC) of a battery system, and more particularly to the use advanced estimation techniques to determine the diffusion in the battery system.
Batteries are used in a wide range of motor vehicle applications. For example, the desire to improve fuel economy and reduce the amount of pollutants emitted by vehicles has led to the development of electric vehicles (EVs) and hybrid electric vehicles (HEVs) incorporating sophisticated battery systems and vehicles propelled by such systems.
Common to all types of electrified or battery-propelled vehicles is a battery that undergoes various charge and discharge cycles as the battery powers the vehicle and/or vehicular components. It is important to know the state of the battery as expressed by the SOC of the battery. In order to make decisions regarding the usage and utilization of the battery it is important to know the SOC of the battery.
The SOC of a battery indicates the amount of usable energy stored within the battery at a given time. It is analogous to the amount of fuel in a fuel tank. In order to improve battery life and overall battery performance, the battery must be maintained at an optimal SOC. The state of health (SOH) of a battery indicates the condition of the battery. The SOH is based on observing particular battery parameters to determine the extent of unobservable, internal damage or degradation.
A battery can be considered a system that has current and heat flow as inputs and responses of voltage and temperature. The voltage response of the battery to current stimulation can be modeled using a combination of voltage states. These voltage states include an equilibrium potential, voltage resulting from hysteresis effects, the voltage drop resulting from Ohmic resistance, and voltage drop resulting from dynamics in the battery (double-layer and/or diffusion voltage). Each of these voltages can be described by an algebraic function, a differential equation or a convolution integral.
During use (either by driving or plug-in-charging), battery resistance, equilibrium potential, voltage resulting from hysteresis effects, voltage drop resulting from Ohmic resistance, double-layer voltage, and diffusion voltage are not directly measurable. These values influence the SOC and the parameters that indicate the SOH of the battery. Because they are not directly measurable, it is often difficult to accurately determine the SOC and SOH of the battery. Therefore, it is difficult to maintain the battery at an optimal SOC or to determine when the SOH has degraded to a point where battery performance is significantly affected.
Different methods have been developed to determine the SOC. One method to determine the SOC is based upon the use of an equivalent circuit of a battery system to model the battery. The equivalent circuit that models the battery accounts for factors such as the surface charge on the plates of the battery. While such approaches provide a model for determining the SOC of the battery, these circuits fail to account for diffusion in the battery system.